Nice - your really loving that thing
I go through phases of not using the V-synth much to the point of thinking I should sell it - then suddenly I need something that only a V-synth will do for then Im reminded again what an amazing tool it is
I suspect the Radias variable filter is actually a careful blend of pairs of filters rather than a true variable filter algorythm.
There are general (and quite complex) algorythms to model any basic filter type which if fed with the right params would allow such modelling, but between LP and HP you should get a BP, or at least a Band Boost if the reso is up which isnt the way the radias works - its blend from LP to HP sounds more like a cross fade than a morph - I dont get the feel of it preserving a band boost over the transition.
The headache with general algorythms that I have seen (within my limited knowledge of the math) is that alot of what we appreciate as 'character' in a filter cant easily be modelled in such a general algorythm (real differences in behaviour between LP, BP and HP modes) - hence why it tends to find a place only in audio processing rather than synthesis.
What is nice about the radias filter vs the virus filter is the model they are using doesnt seem to be the usual stakc a pair of LP12s to get the LP24 which usually has a phase delay penalty with older (more analog like) algorythms. You can notice this on the virus with the reso cranked on certain pair of filter types and oddly they sound seems to cancel out - especially with reso boosted. The radias doesnt exhibit that at all which is nice so some of he black holes in the virus are easily acheiveable on the radias.
Merlot/ten - if you're reading this - whats you're thoughts on the radias filter sweeps?
If you have a mind to its easy to re-create this on a virus with the filter pairs and filter blend - in fact I think some preset patches allready do this between LP and HP.
There are some presets on the V-Synth that seem to do this as well - but havnt bothered to look into what they actually do.